Golf ball

ABSTRACT

An object of the present invention is to provide a golf ball traveling a great flight distance on driver shots. The present invention provides a golf ball having a spherical core and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition containing (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) a specific organic peroxide, and (d) a carboxylic acid and/or a salt thereof, provided that the rubber composition further contains (e) a metal compound in the case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent.

FIELD OF THE INVENTION

The present invention relates to a golf ball having an excellent flying performance, in particular, an improvement of a core of a golf ball.

DESCRIPTION OF THE RELATED ART

As a method for improving a flight distance on driver shots, for example, there are methods of enhancing resilience of a core and controlling a hardness distribution of a core. The former method has an effect of enhancing an initial speed, and the latter method has an effect of a lower spin rate. A golf ball having a low spin rate travels a great distance.

For example, Japanese Patent No. 3674679, No. 3672016, and Japanese Patent Publications No. 2012-139415 A and 2012-192158 A disclose a technique to control a hardness distribution of the core. Japanese Patent No. 3674679 and No. 3672016 disclose a multi-piece golf ball comprising a solid core, wherein the solid core is formed from a rubber composition containing a base rubber; a crosslinking agent; and a mixture of 2,5-dimethyl-2,5-di-t-butylperoxyhexyne-3 and 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane as an organic peroxide in an amount ranging from 0.1 part by mass to 5 parts by mass with respect to the base rubber, and wherein the core has a maximum hardness at a position spaced 3 to 100 mm radially inward from a surface thereof, and a hardness difference between the maximum hardness and a hardness at the center position of 3 or more in JIS-C hardness.

Japanese Patent Publication No. 2012-139415 A discloses a golf ball having a spherical core and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition containing (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) a crosslinking initiator, (d) a salt of a carboxylic acid, and (e) an organic sulfur compound, provided that the rubber composition further contains (f) a metal compound in the case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent, wherein the rubber composition contains (d) the salt of the carboxylic acid in a content of 10 parts by mass or more and less than 40 parts by mass with respect to 100 parts by mass of (a) the base rubber.

Japanese Patent Publication No. 2012-192158 A discloses a golf ball having a spherical core and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition containing (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) a crosslinking initiator, (d) a carboxylic acid and (e) an organic sulfur compound, provided that the rubber composition further contains (f) a metal compound in the case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a golf ball showing an excellent flying performance.

The present invention provides a golf ball having a spherical core and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition containing (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) an organic peroxide represented by a following formula (1), and (d) a carboxylic acid and/or a salt thereof, provided that the rubber composition further contains (e) a metal compound in the case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent.

[In the formula (1), R¹ to R¹⁰ each represent a hydrogen atom, alkyl group, aryl group, aralkyl group, or alkylaryl group independently, wherein the alkyl group, aryl group, aralkyl group, or alkylaryl group may be bonded via an ester group.]

Since the golf ball of the present invention is constructed as described above, the spherical core has a higher degree of an outer-hard inner-soft structure where a surface hardness thereof is higher than a center hardness and a lowered hardness around 37.5% position from a center of a core radius. The golf ball having the spherical core with a higher degree of the outer-hard inner-soft structure exhibits a lower spin rate on driver shots. Further, lower hardness around 37.5% position from the center of the core radius further reduces the spin rate on driver shots. Accordingly, it is expected that the golf ball of the present invention travels an even greater distance on driver shots.

In the present invention, the action of (d) the carboxylic acid and/or a salt thereof in the rubber composition is considered as follows. It is considered that the metal salt of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms blended in the rubber composition forms an ion cluster in the core, resulting in a metal crosslinking of a rubber molecular chain. By blending (d) the carboxylic acid and/or the salt thereof in the rubber composition, (d) the carboxylic acid and/or the salt thereof exchanges a cation with the ion cluster formed by the metal salt of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, thereby breaking the metal crosslinking by (b) the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. The cation exchange reaction easily occurs at the core central part where the temperature is high, and less occurs toward the core surface. When molding the core, the internal temperature of the core is high at the core central part and decreases toward the core surface, since reaction heat from a curing reaction of the rubber composition accumulates at the core central part. That is, the breaking of the metal crosslinking by (d) the carboxylic acid and/or the salt thereof easily occurs at the core central part, but less occurs toward the surface. As a result, it is conceivable that since a crosslinking density in the core increases from the center of the core toward the surface thereof, the hardness increases from the center of the core toward the surface thereof. Further, it is considered that blending (c) the specific organic peroxide having a triple bond in a molecule thereof in addition to (d) the carboxylic acid and/or the salt thereof lowers the hardness around 37.5% position from the center of the core radius while maintaining the outer-hard inner soft structure of the core, due to the specific structure of (c) the organic peroxide.

The present invention provides a golf ball showing an excellent flying performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway sectional view showing the golf ball according to the preferable embodiment of the present invention;

FIG. 2 is a graph showing the hardness distribution of the spherical core;

FIG. 3 is a graph showing the hardness distribution of the spherical core;

FIG. 4 is a graph showing the hardness distribution of the spherical core;

FIG. 5 is a graph showing the hardness distribution of the spherical core;

FIG. 6 is a graph showing the hardness distribution of the spherical core;

FIG. 7 is a graph showing the hardness distribution of the spherical core;

FIG. 8 is a graph showing the hardness distribution of the spherical core;

FIG. 9 is a graph showing the hardness distribution of the spherical core;

FIG. 10 is a graph showing the hardness distribution of the spherical core;

FIG. 11 is a graph showing the hardness distribution of the spherical core;

FIG. 12 is a graph showing the hardness distribution of the spherical core;

FIG. 13 is a graph showing the hardness distribution of the spherical core;

FIG. 14 is a graph showing the hardness distribution of the spherical core;

FIG. 15 is a graph showing the hardness distribution of the spherical core;

FIG. 16 is a graph showing the hardness distribution of the spherical core;

FIG. 17 is a graph showing the hardness distribution of the spherical core;

FIG. 18 is a graph showing the hardness distribution of the spherical core;

FIG. 19 is a graph showing the hardness distribution of the spherical core;

FIG. 20 is a graph showing the hardness distribution of the spherical core;

FIG. 21 is a graph showing the hardness distribution of the spherical core;

FIG. 22 is a graph showing the hardness distribution of the spherical core;

FIG. 23 is a graph showing the hardness distribution of the spherical core;

FIG. 24 is a graph showing the hardness distribution of the spherical core.

FIG. 25 is a graph showing the hardness distribution of the spherical core;

FIG. 26 is a graph showing the hardness distribution of the spherical core;

FIG. 27 is a graph showing the hardness distribution of the spherical core;

FIG. 28 is a graph showing the hardness distribution of the spherical core;

FIG. 29 is a graph showing the hardness distribution of the spherical core;

FIG. 30 is a graph showing the hardness distribution of the spherical core;

FIG. 31 is a graph showing the hardness distribution of the spherical core; and

FIG. 32 is a graph showing the hardness distribution of the spherical core.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a golf ball having a spherical core and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition containing (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) an organic peroxide represented by a following formula (1), and (d) a carboxylic acid and/or a salt thereof, provided that the rubber composition further contains (e) a metal compound in the case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent.

[In the formula (1), R¹ to R¹⁰ each represent a hydrogen atom, alkyl group, aryl group, aralkyl group, or alkylaryl group independently, wherein the alkyl group, aryl group, aralkyl group, or alkylaryl group may be bonded via an ester group.]

First, (a) the base rubber used in the present invention will be explained. As (a) the base rubber used in the present invention, natural rubber and/or synthetic rubber can be used. For example, polybutadiene rubber, natural rubber, polyisoprene rubber, styrene polybutadiene rubber, ethylene-propylene-diene rubber (EPDM), or the like can be used. These rubbers may be used solely or two or more of these rubbers may be used in combination. Among them, typically preferred is the high cis-polybutadiene having a cis-1,4 bond in a proportion of 40% or more, more preferably 80% or more, even more preferably 90% or more in view of its superior resilience property.

The high-cis polybutadiene preferably has a 1,2-vinyl bond in a content of 2 mass % or less, more preferably 1.7 mass % or less, and even more preferably 1.5 mass % or less. If the content of 1,2-vinyl bond is excessively high, the resilience may be lowered.

The high-cis polybutadiene preferably includes one synthesized using a rare earth element catalyst. When a neodymium catalyst, which employs a neodymium compound of a lanthanum series rare earth element compound, is used, a polybutadiene rubber having a high content of a cis-1,4 bond and a low content of a 1,2-vinyl bond is obtained with excellent polymerization activity. Such a polybutadiene rubber is particularly preferred.

The high-cis polybutadiene preferably has a Mooney viscosity (ML₁₊₄ (100° C.)) of 30 or more, more preferably 32 or more, even more preferably 35 or more, and preferably has a Mooney viscosity (ML₁₊₄ (100° C.)) of 140 or less, more preferably 120 or less, even more preferably 100 or less, and most preferably 80 or less. It is noted that the Mooney viscosity (ML₁₊₄ (100° C.)) in the present invention is a value measured according to JIS K6300 using an L rotor under the conditions of: a preheating time of 1 minute; a rotor revolution time of 4 minutes; and a temperature of 100° C.

The high-cis polybutadiene preferably has a molecular weight distribution Mw/Mn (Mw: weight average molecular weight, Mn: number average molecular weight) of 2.0 or more, more preferably 2.2 or more, even more preferably 2.4 or more, and most preferably 2.6 or more, and preferably has a molecular weight distribution Mw/Mn of 6.0 or less, more preferably 5.0 or less, even more preferably 4.0 or less, and most preferably 3.4 or less. If the molecular weight distribution (Mw/Mn) of the high-cis polybutadiene is excessively low, the processability may deteriorate. If the molecular weight distribution (Mw/Mn) of the high-cis polybutadiene is excessively high, the resilience may be lowered. It is noted that the measurement of the molecular weight distribution is conducted by gel permeation chromatography (“HLC-8120GPC”, manufactured by Tosoh Corporation) using a differential refractometer as a detector under the conditions of column: GMHHXL (manufactured by Tosoh Corporation), column temperature: 40° C., and mobile phase: tetrahydrofuran, and calculated by converting based on polystyrene standard.

Next, (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof will be explained. (b) The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof is blended as a co-crosslinking agent in the rubber composition and has an action of crosslinking a rubber molecule by graft polymerization to a base rubber molecular chain. In the case that the rubber composition used in the present invention contains only the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent, the rubber composition further contains (e) the metal compound as an essential component. Neutralizing the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with the metal compound in the rubber composition provides substantially the same effect as using the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. Further, in the case of using the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and the metal salt thereof in combination, (e) the metal compound may be used as an optional component.

The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms includes, for example, acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid, and the like.

Examples of the metals constituting the metal salts of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms include: monovalent metal ions such as sodium, potassium, lithium or the like; divalent metal ions such as magnesium, calcium, zinc, barium, cadmium or the like; trivalent metal ions such as aluminum or the like; and other metal ions such as zirconium or the like. The above metal ions can be used solely or as a mixture of at least two of them. Among these metal ions, divalent metal ions such as magnesium, calcium, zinc, barium, cadmium or the like are preferable. Use of the divalent metal salts of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms easily generates a metal crosslinking between the rubber molecules. Especially, as the divalent metal salt, zinc acrylate is preferable, because the zinc acrylate enhances the resilience of the resultant golf ball. The α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof may be used solely or in combination at least two of them.

The content of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof is preferably 15 parts by mass or more, more preferably 20 parts by mass or more, and is preferably 50 parts by mass or less, more preferably 45 parts by mass or less, even more preferably 35 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the content of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof is less than 15 parts by mass, the content of (c) the crosslinking initiator which will be explained below must be increased in order to obtain the appropriate hardness of the constituting member formed from the rubber composition, which tends to cause the lower resilience. On the other hand, if the content of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof exceeds 50 parts by mass, the constituting member formed from the rubber composition becomes excessively hard, which tends to cause the lower shot feeling.

(c) The organic peroxide represented by the following formula (1) serves as a crosslinking initiator. The crosslinking initiator is blended in order to crosslink (a) the base rubber component.

[In the formula (1), R¹ to R¹⁰ each represent a hydrogen atom, alkyl group, aryl group, aralkyl group, or alkylaryl group independently, wherein the alkyl group, aryl group, aralkyl group, or alkylaryl group may be bonded via an ester group.]

The alkyl group represented by R¹ to R¹⁰ in the formula includes normal alkyl groups such as a methyl group, ethyl group, n-propyl group, n-butyl group, n-pentyl group, n-hexyl group, n-heptyl group, n-octyl group, n-nonyl group, n-decyl group, or the like; branched alkyl groups such as an isopropyl group, isobutyl group, tert-butyl group, isopentyl group, neopentyl group, 2-ethylhexyl group, and isooctyl group; cycloalkyl groups such as cyclopropyl group, cyclobutyl group, cyclopentyl group, cyclohexyl group, and cycloheptyl group. Among them, preferred is an alkyl group having 1 to 11 carbon atoms, and more preferred is an alkyl group having 1 to 6 carbon atoms, even more preferred is methyl group or ethyl group. Further, the normal alkyl group is preferable as an alkyl group.

The aryl group represented by R¹ to R¹⁰ in the formula includes a phenyl group, naphthyl group, anthryl group, biphenyl group, phenanthryl group, fluorenyl group or the like, and among them, a phenyl group is preferred. The aralkyl group represented by R¹ to R¹⁰ in the formula includes a benzyl group, phenethyl group, phenylpropyl group, naphthylmethyl group, naphthylethyl group or the like. The alkylaryl group represented by R¹ to R¹⁰ in the formula includes a tolyl group, xylyl group, cumenyl group, mesityl group or the like.

R¹, R⁵, R⁶, and R¹⁹ preferably include an alkyl group having 1 to 11 carbon atoms, more preferably a methyl group or ethyl group. R² to R⁴ and R⁷ to R⁹ preferably include an aryl group or alkyl group having 1 to 11 carbon atoms, more preferably the aryl group or alkyl group having 1 to 6 carbon atoms, even more preferably a methyl group, ethyl group or phenyl group.

Specific examples of (c) the organic peroxide include 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3; 3,6-dimethyl-3,6-di(t-butylperoxy)octyne-4; 2,5-dimethyl-2-[(triphenylsilyl)peroxy]-5-(t-butylperoxy)hexyne-3; 2,5-dimethyl-2,5-[(triphenylsilyl)peroxy]hexyne-3. (c) The organic peroxide may be used alone or in combination at least two of them.

The content of (c) the crosslinking initiator is preferably 0.1 part by mass or more, and more preferably 0.2 part by mass or more, and is preferably 5.0 parts by mass or less, and more preferably 3.0 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the content of (c) the crosslinking initiator is less than 0.1 part by mass, the constituting member formed from the rubber composition becomes too soft, and thus the golf ball may have the lower resilience. If the content of (c) the crosslinking initiator exceeds 5.0 parts by mass, the amount of (b) the co-crosslinking agent must be decreased in order to obtain the appropriate hardness of the constituting member formed from the rubber composition, resulting in the insufficient resilience and lower durability of the golf ball.

The rubber composition used in the present invention may further comprise another crosslinking initiator in addition to (c) the organic peroxide. As another crosslinking initiator, an organic peroxide other than (c) the organic peroxide is preferred. Specific examples of another organic peroxide include organic peroxides such as dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. These organic peroxides may be used solely or two or more of these organic peroxides may be used in combination. In the case of using another crosslinking initiator, the content of (C) the organic peroxide is preferably 60 mass % or more, more preferably 70 mass % or more, even more preferably 80 mass % or more in a total of 100% of (c) the organic peroxide and another crosslinking agent. As the crosslinking initiator, it is preferable that only (c) the organic peroxide is blended.

Next, (d) the carboxylic acid and/or the salt thereof will be explained. It is conceivable that (d) the carboxylic acid and/or the salt thereof has an action of breaking the metal crosslinking by the metal salt of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms at the center part of the core, when molding the core. (d) The carboxylic acid and/or the salt thereof includes, for example, an aliphatic carboxylic acid, a salt of an aliphatic carboxylic acid, an aromatic carboxylic acid, and a salt of an aromatic carboxylic acid. In (d) the carboxylic acid and/or the salt thereof, (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof used as the co-crosslinking agent should not be included. (d) The carboxylic acid and/or the salt thereof may be used alone or in combination of at least two of them.

(d) The aliphatic carboxylic acid and/or the salt thereof preferably includes an aliphatic carboxylic acid having 1 to 30 carbon atoms, more preferably an aliphatic carboxylic acid having 1 to 18 carbon atoms, even more preferably an aliphatic carboxylic acid having 1 to 13 carbon atoms.

The aliphatic carboxylic acid may be either a saturated fatty acid or an unsaturated fatty acid. Further, the aliphatic carboxylic acid may have a branched structure or a cyclic structure. Specific examples of the aliphatic carboxylic acid (IUPAC name) are methanoic acid (C1), ethanoic acid (C2), propanoic acid (C3), butanoic acid (C4), pentanoic acid (C5), hexanoic acid (C6), heptanoic acid (C7), octenoic acid (C8), nonanoic acid (C9), decanoic acid (C10), undecanoic acid (C11), dodecanoic acid (C12), tridecanoic acid (C13), tetradecanoic acid (C14), pentadecanoic acid (C15), hexadecanoic acid (C16), heptadecanoic acid (C17), octadecanoic acid (C18), nonadecanoic acid (C19), icosanoic acid (C20), henicosenoic acid (C21), docosenoic acid (C22), tricosanoic acid (C23), tetracosanoic acid (C24), pentacosanoic acid (C25), hexacosanoic acid (C26), heptacosanoic acid (C27), octacosanoic acid (C28), nonacosanoic acid (C29), and triacontanoic acid (C30).

Specific examples of the unsaturated fatty acid (IUPAC name) are etenoic acid (C2), propenoic acid (C3), butenoic acid (C4), pentenoic acid (C5), hexenoic acid (C6), heptenoic acid (C7), octenoic acid (C8), nonenoic acid (C9), decenoic acid (C10), undecenoic acid (C11), dodecenoic acid (C12), tridecenoic acid (C13), tetradecenoic acid (C14), pentadecenoic acid (C15), hexadecenoic acid (C16), heptadecenoic acid (C17), octadecenoic acid (C18), nonadecenoic acid (C19), icosenoic acid (C20), henicosenoic acid (C21), docosenoic acid (C22), tricosenoic acid (C23), tetracosenoic acid (C24), pentacosenoic acid (C25), hexacosenoic acid (C26), heptacosenoic acid (C27), octacosenoic acid (C28), nonacosenoic acid (C29), and triacontanoic acid (C30).

Specific examples (common name) of the fatty acids are formic acid (C1), acetic acid (C2), propionic acid (C3), butyric acid (C4), valeric acid (C5), caproic acid (C6), enanthic acid (C7), caprylic acid (C8), pelargonic acid (C9), capric acid (C10), lauric acid (C12), myristic acid (C14), myristoleic acid (C14), pentadecylic acid (C15), palmitic acid (C16), palmitoleic acid (C16), margaric acid (C17), stearic acid (C18), elaidic acid (C18), vaccenic acid (C18), oleic acid (C18), linoleic acid (C18), linolenic acid (C18), 12-hydroxystearic acid (C18), arachidic acid (C20), gadoleic acid (C20), arachidonic acid (C20), eicosenoic acid (C20), behenic acid (C22), erucic acid (C22), lignoceric acid (C24), nervonic acid (C24), cerotic acid (C26), montanic acid (C28), and melissic acid (C30).

The aromatic carboxylic acid includes, for example, a carboxylic acid having a benzene ring in a molecule thereof, a carboxylic acid having a heteroaromatic ring in a molecule thereof and the like.

(d) The carboxylic acid having a benzene ring includes, for example, an aromatic carboxylic acid having a carboxyl group directly bonding to the benzene ring, an aromatic-aliphatic carboxylic acid where an aliphatic carboxylic acid is bonded to the benzene ring, a polynuclear aromatic carboxylic acid having a carboxyl group directly bonding to the fused benzene ring, and a polynuclear aromatic-aliphatic carboxylic acid where an aliphatic carboxylic acid is bonded to the fused benzene ring. The fused benzene ring structure includes, for example, naphthalene, anthracene, phenalene, phenanthrene, tetracene, and pyrene.

The number of the carboxylic group of (d) the carboxylic acid having the benzene ring may be either one (monocarboxylic acid) or at least two (polycarboxylic acid), but preferably one. The benzene ring or fused benzene ring may have another substituent group directly bonding to the benzene ring or fused benzene ring than the carboxylic group. Such a substituent group includes, for example, an alkyl group (preferably, alkyl group having 1 to 4 carbon atoms), aryl group (preferably, phenyl group), amino group, hydroxyl group, alkoxyl group (preferably, alkoxyl group having 1 to 4 carbon atoms), oxo group, and halogen group.

Specific examples of the aromatic carboxylic acid having a carboxylic acid directly bonding to the benzene ring, include, for example, benzoic acid (C7), phthalic acid (C8), isophthalic acid (C8), terephthalic acid (C8), benzene-1,2,3-tricarboxylic acid (C9), benzene-1,2,4-tricarboxylic acid (C9), benzene-1,3,5-tricarboxylic acid (C9), benzene-1,2,3,4-tetracarboxylic acid (C10), benzene-1,2,3,5-tetracarboxylic acid (C10), benzene-1,2,4,5-tetracarboxylic acid (C10), and benzene hexacarboxylic acid (C12). Specific examples of the aromatic-aliphatic carboxylic acid where the aliphatic carboxylic acid is bonded to the benzene ring, includes phenylacetic acid (C8), 2-phenylpropanoic acid (C9), and 3-phenylpropanoic acid (C9).

Furthermore, examples of the carboxylic acid having a benzene ring substituted with an alkyl group, aryl group, amino group, hydroxyl group, alkoxy group, or oxo group include, for example, methylbenzoic acid (C8), dimethylbenzoic acid (C9), 2,3,4-trimethylbenzoic acid (C10), 2,3,5-trimethylbenzoic acid (C10), 2,4,5-triemethylbenzoic acid (C10), 2,4,6-trimethylbenzoic acid (C10), 3,4,5-trimethylbenzoic acid (C10), 4-isopropylbenzoic acid (C10), 4-tert-butylbenzoic acid (C11), 5-methylisophthalic acid (C9), biphenyl-4-carboxylic acid (C13), biphenyl-2,2′-dicarboxylic acid (C14), 4-dimethylaminobenzoic acid (C9), 2-hydroxybenzoic acid (C7), methoxybenzoic acid (C8), hydroxy(methyl)benzoic acid (C8), 2-hydroxy-3-methylbenzoic acid (C8), 2-hydroxy-4-methylbenzoic acid (C8), 2-hydroxy-5-methylbenzoic acid (C8), 2,3-dihydroxybenzoic acid (C7), 2,4-dihydroxybenzoic acid (C7), 2,6-dihydroxybenzoic acid (C7), 3,4-dihydroxybenzoic acid (C7), 3,5-dihydroxybenzoic acid (C7), 4-hydroxy-3-methoxybenzoic acid (C8), 3-hydroxy-4-methoxybenzoic acid (C8), 3,4-dimethoxybenzoic acid (C9), 2,3-dimethoxybenzoic acid (C9), 2,4-dimethoxbenzoic acid (C9), 2,4-dihydroxy-6-methylbenzoic acid (C8), 4,5-dimethoxyphthalic acid (C10), 3,4,5-trihydroxybenzoic acid (C7), 4-hydroxy-3,5-dimethoxybenzoic acid (C9), 2,4,5-trimethoxybenzoic acid (C10), hydroxy(phenyl)acetic acid (C8), hydroxy(4-hydroxy-3-methoxyphenyl)acetic acid (C9), (4-methoxyphenyl)acetic acid (C9), (2,5-dihydroxyphenyl)acetic acid (C8), (3,4-dihydroxyphenyl)acetic acid (C8), (4-hydroxy-3-methoxyphenyl)acetic acid (C9), (3-hydroxy-4-methoxyphenyl)acetic acid (C9), (3,4-dimethoxyphenyl)acetic acid (C10), (2,3-dimethoxyphenyl)acetic acid (C10), 2-(carboxymethyl)benzoic acid (C9), 3-(carboxymethyl)benzoic acid (C9), 4-(carboxymethyl)benzoic acid (C9), 2-(carboxycarbonyl)benzoic acid (C9), 3-(carboxycarbonyl)benzoic acid (C9), 4-(carboxycarbonyl)benzoic acid (C9), 2-hydroxy-2-phenylpropanoic acid (C9), 3-hydroxy-2-phenylpropanoic acid (C9), 3-(2-hydroxyphenyl)propanoic acid (C9), 3-(4-hydroxyphenyl)propanoic acid (C9), 3-(3,4-dihydroxyphenyl)propanoic acid (C9), 3-(4-hydroxy-3-methoxyphenyl)propanoic acid (C10), 3-(3-hydroxy-4-methoxyphenyl)propanoic acid (C10), 3-(4-hydroxyphenyl)acrylic acid (C9), 3-(2,4-dihydroxyphenyl)acrylic acid (C9), 3-(3,4-dihydroxyphenyl)acrylic acid (C9), 3-(4-hydroxy-3-methoxyphenyl)acrylic acid (C10), 3-(3-hydroxy-4-methoxyphenyl) acrylic acid (C10), and 3-(4-hydroxy-3,5-dimethoxyphenyl)acrylic acid (C11).

The carboxylic acid having the benzene ring substituted with halogen includes, for example, carboxylic acids where at least one hydrogen of benzoic acid is substituted with a fluoro group such as fluorobenzoic acid, difluorobenzoic acid, trifluorobenzoic acid, tetrafluorobenzoic acid, and pentafluorobenzoic acid; carboxylic acids where at least one hydrogen of benzoic acid is substituted with a chloro group such as chlorobenzoic acid, dichlorobenzoic acid, trichlorobenzoic acid, tetrachlorobenzoic acid, and pentachlorobenzoic acid; carboxylic acids where at least one hydrogen of benzoic acid is substituted with a bromo group such as bromobenzoic acid, dibromobenzoic acid, tribromobenzoic acid, tetrabromobenzoic acid, and pentabromobenzoic acid; and carboxylic acids where at least one hydrogen of benzoic acid is substituted with a iodo group such as iodobenzoic acid, diiodobenzoic acid, triiodobenzoic acid, tetraiodobenzoic acid, and pentaiodobenzoic acid.

Specific examples of the polynuclear aromatic carboxylic acid having a carboxyl group directly bonding to the fused benzene ring include 1-naphthalene carboxylic acid, 2-naphthalene carboxylic acid, 1-anthracene carboxylic acid, 2-anthracene carboxylic acid, 9-anthracene carboxylic acid, phenanthrene carboxylic acid, and pyrene carboxylic acid. Specific examples of the polynuclear aromatic-aliphatic carboxylic acid where the aliphatic carboxylic acid is bonded to the fused benzene ring include naphthylacetic acid, and naphthylpropionic acid.

The carboxylic acid having a fused benzene ring substituted with halogen includes, for example, fluoronaphthalene carboxylic acid, chloronaphthalene carboxylic acid, bromonaphthalene carboxylic acid, fluoroanthracene carboxylic acid, chloroanthracene carboxylic acid, and bromoanthracene carboxylic acid. The carboxylic acid having a heteroaromatic ring includes, for example, a carboxylic acid where a carboxylic acid is directly bonded to the heteroaromatic ring. The hetero atom in the heteroaromatic ring can be one kind or two or more kinds. The hetero atom includes a nitrogen atom, oxygen atom, sulfur atom or the like. Among them, the oxygen atom or sulfur atom is preferred. The number of the hetero atom in the heteroaromatic ring is not particularly limited, but preferably 2 or less, and more preferably 1. The heteroaromatic ring includes, for example, a pyrrole ring, furan ring, thiophene ring, imidazole ring, pyrazole ring, oxazole ring, thiazole ring, pyridine ring, pyrazine ring, indole ring, quinolone ring, benzofuran ring, and benzothiophene ring.

(d) The carboxylic acid having a heteroaromatic ring may be a compound having only a carboxyl group as a substituent group to the heteroaromatic ring, and a compound having another substituent group directly bonding to the heteroaromatic ring in addition to the carboxyl group. Further, the substituent group may bond to a nitrogen atom constituting the heteroaromatic ring. The substituent group includes, for example, halogen, a hydroxyl group, a mercapto group, an alkyl group, an aryl group, an aralkyl group, an alkylaryl group, an alkoxyl group, an amino group which may be substituted, a cyano group, or a thiocarboxyl group.

Specific examples of the carboxylic acid having the heteroaromatic ring and/or the salt thereof include, carboxylic acids having a five-membered heteroaromatic ring such as a pyrrole carboxylic acid, furan carboxylic acid, thiophene carboxylic acid, imidazole carboxylic acid, pyrazole carboxylic acid, oxazole carboxylic acid, and thiazole carboxylic acid; carboxylic acids having a six-membered heteroaromatic ring such as a pyridine carboxylic acid, and a pyrazine carboxylic acid; and carboxylic acids having a fused heteroaromatic ring such as an indolecarboxylic acid, quinolinecarboxylic acid, benzofuran carboxylic acid, and benzothiophene carboxylic acid.

As (d) the salt of the aliphatic carboxylic acid or aromatic carboxylic acid, a salt of the aliphatic carboxylic acid or aromatic carboxylic acid described above may be used. The cation component of the salt of these carboxylic acids may be any one of a metal ion, an ammonium ion and an organic cation. The metal ion includes monovalent metal ions such as sodium, potassium, lithium, silver and the like; divalent metal ions such as magnesium, calcium, zinc, barium, cadmium, copper, cobalt, nickel, manganese and the like; trivalent metal ions such as aluminum, iron and the like; and other ions such as tin, zirconium, titanium and the like. The cation components may be used alone or as a mixture of at least two of them. The cation component preferably includes a zinc ion.

The organic cation includes a cation having a carbon chain. The organic cation includes, for example, without limitation, an organic ammonium ion. Examples of the organic ammonium ion are: primary ammonium ions such as stearyl ammonium ion, hexyl ammonium ion, oethyl ammonium ion, 2-ethyl hexyl ammonium ion or the like; secondary ammonium ions such as dodecyl(lauryl) ammonium ion, octadecyl(stearyl) ammonium ion or the like; tertiary ammonium ions such as trioctyl ammonium ion or the like; and quaternary ammonium ions such as dioctyldimethyl ammonium ion, distearyldimethyl ammonium ion or the like. These organic cation may be used alone or as a mixture of at least two of them.

(d) The aliphatic carboxylic acid and/or the salt thereof preferably includes a saturated fatty acid and/or the salt thereof. Preferable examples thereof include caprylic acid (octanoic acid), pelargonic acid (nonanoic acid), capric acid (decanoic acid), lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, and oleic acid, or a potassium salt, magnesium salt, calcium salt, aluminum salt, zinc salt, iron salt, copper salt, nickel salt, or cobalt salt of the above aliphatic carboxylic acids. (d) The aromatic carboxylic acid and/or the salt thereof preferably includes benzoic acid, butylbezoic acid, anisic acid (methoxybenzoic acid), dimethoxybenzoic acid, trimethoxybenzoic acid, dimethylaminobenzoic acid, chlorobenzoic acid, dichlorobenzoic acid, trichlorobenzoic acid, acetoxybenzoic acid, biphenyl carboxylic acid, naphthalene carboxylic acid, anthracene carboxylic acid, furan carboxylic acid, and thenoyl carboxylic acid, or a potassium salt, magnesium salt, calcium salt, aluminum salt, zinc salt, iron salt, copper salt, nickel salt, or cobalt salt of the above aromatic carboxylic acids.

The content of (d) the carboxylic acid and/or the salt thereof is preferably 0.5 part by mass or more, more preferably 1.0 parts by mass or more, even more preferably 1.5 parts by mass or more, and is preferably 40 parts by mass or less, more preferably 35 parts by mass or less, even more preferably 30 parts by mass or less with respect to 100 parts by mass of (a) the base rubber. If the content is too little, the effect of adding (d) the carboxylic acid and/or the salt thereof is not sufficient, and thus the degree of the outer-hard inner-soft structure of the spherical core may be lowered. If the content is too much, the resilience of the core may be lowered, since the hardness of the resultant core may be lowered as a whole.

There are cases where the surface of the zinc acrylate used as the co-crosslinking agent is treated with the carboxylic acid and/or the salt thereof to improve the dispersibility to the rubber. In the case of using zinc acrylate whose surface is treated with the carboxylic acid and/or the salt thereof, in the present invention, the amount of the carboxylic acid and/or the salt thereof used as a surface treating agent is included in the content of (d) the carboxylic acid and/or the salt thereof. For example, if 25 parts by mass of zinc acrylate whose surface treatment amount with the carboxylic acid and/or the salt thereof is 10 mass % is used, the amount of the carboxylic acid and/or the salt thereof is 2.5 parts by mass and the amount of zinc acrylate is 22.5 parts by mass. Thus, 2.5 parts by mass is counted as the content of (d) the carboxylic acid and/or the salt thereof.

In the case that the rubber composition used in the present invention contains only the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent, the rubber composition further contains (e) a metal compound as an essential component. (e) The metal compound is not limited as long as it can neutralize (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms in the rubber composition. (e) The metal compound includes, for example, metal hydroxides such as magnesium hydroxide, zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, copper hydroxide, and the like; metal oxides such as magnesium oxide, calcium oxide, zinc oxide, copper oxide, and the like; metal carbonates such as magnesium carbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithium carbonate, potassium carbonate, and the like. Among these, (e) the metal compound preferably includes a divalent metal compound, more preferably includes a zinc compound. The divalent metal compound reacts with the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, thereby forming a metal crosslinking. Use of the zinc compound provides a golf ball with excellent resilience. (e) These metal compounds are used solely or as a mixture of at least two of them. The content of (e) the metal compound may be adjusted depending upon the desired neutralization degree of (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

The rubber composition used in the present invention preferably further contains (f) an organic sulfur compound. By using (f) the organic sulfur compound in addition to (c) the organic peroxide and (d) the carboxylic acid and/or the salt thereof for the rubber composition, the degree of the outer-hard and inner-soft structure of the core can be controlled at a higher level. (f) The organic sulfur compound can be used alone or in combination of at least two of them.

(f) The organic sulfur compound is not particularly limited, as long as it is an organic compound having a sulfur atom in the molecule thereof. Examples thereof include an organic compound having a thiol group (—SH), a polysulfide bond having 2 to 4 sulfur atoms (—S—S—, —S—S—S—, or —S—S—S—S—), or a metal salt thereof (—SM, —S-M-S—, —S-M-S—S—, —S—S-M-S—S—, —S-M-S—S—S—, or the like; M is a metal atom). Examples of the metal salts are salts of monovalent metals such as sodium, lithium, potassium, copper (I), and silver (I), and salts of divalent metals such as zinc, magnesium, calcium, strontium, barium, titanium (II), manganese (II), iron (II), cobalt (II), nickel(II), zirconium(II), and tin (II). Furthermore, (f) the organic sulfur compound may be any one of aliphatic compounds (aliphatic thiol, aliphatic thiocarboxylic acid, aliphatic dithiocarboxylic acid, aliphatic polysulfides, or the like), heterocyclic compounds, alicyclic compounds (alicyclic thiol, alicyclic thiocarboxylic acid, alicyclic dithiocarboxylic acid, alicyclic polysulfides, or the like), and aromatic compounds.

(f) The organic sulfur compound includes, for example, thiols (thiophenols, thionaphthols), polysulfides, thiurams, thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, dithiocarbamates, and thiazoles.

The thiols preferably include thiophenols and thionaphthols. Examples of the thiophenols include, for example, thiophenol; thiophenols substituted with a fluoro group, such as 4-fluorothiophenol, 2,4-difluorothiophenol, 2,5-difluorothiophenol, 2,6-difluorothiophenol, 2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol, pentafluorothiophenol and the like; thiophenols substituted with a chloro group, such as 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol, 2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol, pentachlorothiophenol and the like; thiophenols substituted with a bromo group, such as 4-bromothiophenol, 2,4-dibromothiophenol, 2,5-dibromothiophenol, 2,6-dibromothiophenol, 2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol, pentabromothiophenol and the like; thiophenols substituted with an iodo group, such as 4-iodothiophenol, 2,4-diiodothiophenol, 2,5-diiodothiophenol, 2,6-diiodothiophenol, 2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol, pentaiodothiophenol and the like.

The metal salt of thiophenols includes, for example, monovalent metal salts of sodium, lithium, potassium, copper (I), silver (I) or the like; and divalent metal salts of zinc, magnesium, calcium, strontium, barium, titanium (II), manganese (II), iron (II), cobalt (II), nickel (II), zirconium (II), tin (II) or the like.

Examples of the thionaphthols (naphthalenethiols) are 2-thionaphthol, 1-thionaphthol, 1-chloro-2-thionaphthol, 2-chloro-1-thionaphthol, 1-bromo-2-thionaphthol, 2-bromo-1-thionaphthol, 1-fluoro-2-thionaphthol, 2-fluoro-1-thionaphthol, 1-cyano-2-thionaphthol, 2-cyano-1-thionaphthol, 1-acetyl-2-thionaphthol, 2-acetyl-1-thionaphthol, and metal salts thereof. Preferable examples include 1-thionaphthol, 2-thionaphthol and metal salt thereof. The metal salt preferably includes a divalent metal salt, more preferably zinc salt. Specific examples of the metal salts include a zinc salt of 1-thionaphthol and a zinc salt of 2-thionaphthol.

Polysulfides are an organic sulfur compound having polysulfide bonds. Examples thereof include disulfides, trisulfides, and tetrasulfides. As the polysulfides, diphenyl polysulfides are preferred.

Examples of the diphenyl polysulfides include diphenyl disulfide; diphenyl disulfides substituted with a fluoro group such as bis(4-fluorophenyl)disulfide, bis(2,4-difluorophenyl)disulfide, bis(2,5-difluorophenyl)disulfide, bis(2,6-difluorophenyl)disulfide, bis(2,4,5-trifluorophenyl)disulfide, bis(2,4,5,6-tetrafluorophenyl)disulfide and bis(pentafluorophenyl)disulfide; diphenyl disulfides substituted with a chloro group such as bis(4-chlorophenyl)disulfide, bis(2,4-dichlorophenyl)disulfide, bis(2,5-dichlorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,4,5-trichlorophenyl)disulfide, bis(2,4,5,6-tetrachlorophenyl)disulfide and bis(pentachlorophenyl)disulfide; diphenyl disulfides substituted with a bromo group such as bis(4-bromophenyl)disulfide, bis(2,4-dibromophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(2,6-dibromophenyl)disulfide, bis(2,4,5-tribromophenyl)disulfide, bis(2,4,5,6-tetrabromophenyl)disulfide and bis(pentabromophenyl)disulfide; diphenyl disulfides substituted with a iodo group such as bis(4-iodophenyl)disulfide, bis(2,4-diiodophenyl)disulfide, bis(2,5-diiodophenyl)disulfide, bis(2,6-diiodophenyl)disulfide, bis(2,4,5-triiodophenyl)disulfide, bis(2,4,5,6-tetraiodophenyl)disulfide and bis(pentaiodophenyl)disulfide; and diphenyl disulfides substituted with an alkyl group such as bis(4-methylphenyl)disulfide, bis(2,4,5-trimethylphenyl)disulfide, bis(pentamethylphenyl)disulfide, bis(4-tert-butylphenyl)disulfide, bis(2,4,5-tri-t-butylphenyl)disulfide, and bis(pent-t-butylphenyl)disulfide.

The thiurams include, for example, thiuram monosulfide such as tetramethylthiuram monosulfide; thiuram disulfides such as tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide; and thiuram tetrasulfide such as dipentamethylenethiuram tetrasulfide. The thiocarboxylic acids include, for example, naphthalene thiocarboxylic acid. The dithiocarboxylic acid includes, for example, naphthalene dithiocarboxylic acid. The sulfenamides include, for example, N-cyclohexyl-2-benzothiazole sulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, and N-t-butyl-2-benzothiazole sulfenamide.

(f) The organic sulfur compound preferably includes thiophenols and/or metal salts thereof, thionaphthols and/or metal salts thereof, diphenyl disulfides, and thiuram disulfides. Preferable examples thereof include 2,4-dichlorothiophenol, 2,6-difluorotihophenol, 2,6-dichlorothiophenol, 2,6-dibromothiophenol, 2,6-diiodothiophenol, 2,4,5-trichlorothiophenol, pentachlorothiophenol, 1-thionaphthol, 2-thionaphthol, diphenyl disulfide, bis(2,6-difluorophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(2,6-dibromophenyl)disulfide, bis(2,6-iodophenyl)disulfide, and bis(pentabromophenyl)disulfide.

The rubber composition preferably includes at least one member selected from the group consisting of thiophenols substituted with halogen, metal salts of thiophenols substituted with halogen, and diphenyl disulfide substituted with halogen. Blending these specific halogenated organic sulfur compounds enhances the resilience of the core. Halogen preferably includes fluororine, chlorine, bromine, and iodine, more preferably chlorine and bromine.

In the case that the rubber composition contains the above specific halogenated organic sulfur compound, a content of thiophenols substituted with halogen, metal salts of thiophenols substituted with halogen, and diphenyl disulfide substituted with halogen in the all of the organic sulfur compound is 60 mass % or more, more preferably 80 mass % or more, even more preferably 90 mass % or more. Further, it is preferable that the rubber composition contains only at least one member selected from the group consisting of thiophenols substituted with halogen, metal salts of thiophenols substituted with halogen, and diphenyl disulfide substituted with halogen as the organic sulfur compound.

The content of (f) the organic sulfur compound is preferably 0.05 part by mass or more, more preferably 0.1 part by mass or more, and is preferably 5.0 parts by mass or less, more preferably 2.0 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the content of (f) the organic sulfur compound is less than 0.05 part by mass, the effect of adding (f) the organic sulfur compound cannot be obtained and thus the resilience may not improve. If the content of (f) the organic sulfur compound exceeds 5.0 parts by mass, the compression deformation amount of the obtained golf ball becomes large and thus the resilience may be lowered.

A mass ratio ((d)/(f)) of (d) the carboxylic acid and/or a salt thereof to (f) the organic sulfur compound is preferably 2 or more, more preferably 5 or more, even more preferably 10 or more, and is preferably 150 or less, more preferably 100 or less, even more preferably 80 or less.

The rubber composition used in the present invention may include additives such as a pigment, a filler for adjusting weight or the like, an antioxidant, a peptizing agent, and a softener where necessary. The pigment blended in the rubber composition includes, for example, a white pigment, a blue pigment, and a purple pigment.

As the white pigment, titanium oxide is preferably used. The type of titanium oxide is not particularly limited, but rutile type is preferably used because of the high opacity. The blending amount of titanium oxide is preferably 0.5 part by mass or more, and more preferably 2 parts by mass or more, and is preferably 8 parts by mass or less, and more preferably 5 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber.

It is also preferred that the rubber composition contains both a white pigment and a blue pigment. The blue pigment is blended in order to make white color vivid, and examples thereof include ultramarine blue, cobalt blue, and phthalocyanine blue. Examples of the purple pigment include anthraquinone violet, dioxazine violet, and methyl violet.

The blending amount of the blue pigment is preferably 0.001 part by mass or more, and more preferably 0.05 part by mass or more, and is preferably 0.2 part by mass or less, and more preferably 0.1 part by mass or less, with respect to 100 parts by mass of (a) the base rubber. If the blending amount of the blue pigment is less than 0.001 part by mass, blueness is insufficient, and the color looks yellowish. If the blending amount of the blue pigment exceeds 0.2 part by mass, blueness is excessively strong, and a vivid white appearance is not provided.

The filler blended in the rubber composition is used as a weight adjusting agent for mainly adjusting the weight of the golf ball obtained as a final product. The filler may be blended where necessary. The filler includes, for example, inorganic fillers such as zinc oxide, barium sulfate, calcium carbonate, magnesium oxide, tungsten powder, molybdenum powder, or the like. As the filler, preferred is zinc oxide. It is considered that zinc oxide functions as a vulcanization activator and increases the hardness of the entire core. The content of the filler is preferably 0.5 part by mass or more, more preferably 1 part by mass or more, and is preferably 30 parts by mass or less, more preferably 25 parts by mass or less, even more preferably 20 parts by mass or less with respect to 100 parts by mass of the base rubber. If the content of the filler is less than 0.5 part by mass, it is difficult to adjust the weight, while if the content of the filler exceeds 30 parts by mass, the weight ratio of the rubber component is reduced and thus the resilience tends to be lowered.

The blending amount of the antioxidant is preferably 0.1 part by mass or more and 1 part by mass or less, with respect to 100 parts by mass of (a) the base rubber. In addition, the blending amount of the peptizing agent is preferably 0.1 part by mass or more and 5 parts by mass or less, with respect to 100 parts by mass of (a) the base rubber.

The rubber composition used in the present invention is obtained by mixing and kneading (a) the base rubber, (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof, (c) the organic peroxide, (d) the carboxylic acid and/or the salt thereof; and, where necessary, (e) the metal compound, (f) the organic sulfur compound, and other additives. The kneading can be conducted, without any limitation, with a well-known kneading machine such as a kneading roll, a banbury mixer, a kneader, or the like.

The spherical core of the golf ball of the present invention can be obtained by molding the rubber composition after kneaded. The temperature for molding into the spherical core is preferably 120° C. or more, more preferably 150° C. or more, even more preferably 160° C. or more, and is preferably 170° C. or less. If the molding temperature exceeds 170° C., the surface hardness of the core tends to decrease. The molding pressure preferably ranges from 2.9 MPa to 11.8 MPa. The molding time preferably ranges from 10 minutes to 60 minutes.

The spherical core preferably has a hardness difference (Hs-Ho) between a surface hardness Hs and a center hardness Ho of 12 or more, more preferably 16 or more, even more preferably 20 or more, and preferably has a hardness difference of 80 or less, more preferably 70 or less, even more preferably 60 or less in JIS-C hardness. If the hardness difference between the center hardness and the surface hardness is large, the golf ball having a great flight distance due to the high launch angle and low spin rate is obtained.

The spherical core preferably has the center hardness Ho of 30 or more, more preferably 35 or more, even more preferably 40 or more in JIS-C hardness. If the center hardness Ho is less than 30 in JIS-C hardness, the core becomes too soft and thus the resilience may be lowered. Further, the spherical core preferably has the center hardness Ho of 70 or less, more preferably 65 or less, even more preferably 60 or less in JIS-C hardness. If the center hardness Ho exceeds 70 in JIS-C hardness, the core becomes too hard and thus the shot feeling tends to be lowered.

The spherical core preferably has the surface hardness Hs of 65 or more, more preferably 70 or more, and preferably has the surface hardness Hs of 100 or less, more preferably 95 or less in JIS-C hardness. If the surface hardness is 65 or more in JIS-C hardness, the spherical core does not become excessively soft, and thus the better resilience is obtained. Further, if the surface hardness of the spherical core is 100 or less in JIS-C hardness, the spherical core does not become excessively hard, and thus the better shot feeling is obtained.

The spherical core preferably has the diameter of 34.8 mm or more, more preferably 36.8 mm or more, and even more preferably 38.8 mm or more, and preferably has the diameter of 42.2 mm or less, more preferably 41.8 mm or less, and even more preferably 41.2 mm or less, and most preferably 40.8 mm or less. If the spherical core has the diameter of 34.8 mm or more, the thickness of the cover does not become too thick and thus the resilience becomes better. On the other hand, if the spherical core has the diameter of 42.2 mm or less, the thickness of the cover does not become too thin, and thus the cover functions better.

When the spherical core has a diameter from 34.8 mm to 42.2 mm, a compression deformation amount (shrinking deformation amount of the spherical core along the compression direction) of the spherical core when applying a load from 98 N as an initial load to 1275 N as a final load is preferably 2.0 mm or more, more preferably 2.8 mm or more, and is preferably 6.0 mm or less, more preferably 5.0 mm or less. If the compression deformation amount is 2.0 mm or more, the shot feeling of the golf ball becomes better. If the compression deformation amount is 6.0 mm or less, the resilience of the golf ball becomes better.

The golf ball cover of the present invention is formed from a cover composition containing a resin component. Examples of the resin component include, for example, an ionomer resin; a thermoplastic polyurethane elastomer having a commercial name of “Elastollan” commercially available from BASF Japan Ltd; a thermoplastic polyamide elastomer having a commercial name of “Pebax” commercially available from Arkema K. K.; a thermoplastic polyester elastomer having a commercial name of “Hytrel” commercially available from Du Pont-Toray Co., Ltd.; and a thermoplastic styrene elastomer having a commercial name of “Rabalon” commercially available from Mitsubishi Chemical Corporation; and the like.

The ionomer resin includes a product prepared by neutralizing at least a part of carboxyl groups in the binary copolymer composed of an olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms with a metal ion, a product prepared by neutralizing at least a part of carboxyl groups in the ternary copolymer composed of an olefin, an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and an α,β-unsaturated carboxylic acid ester with a metal ion, or a mixture of those. The olefin preferably includes an olefin having 2 to 8 carbon atoms. Examples of the olefin are ethylene, propylene, butene, pentene, hexene, heptene, and octene. The olefin more preferably includes ethylene. Examples of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms are acrylic acid, methacrylic acid, fumaric acid, maleic acid and crotonic acid. Among these, acrylic acid and methacrylic acid are particularly preferred. Examples of the α,β-unsaturated carboxylic acid ester include methyl ester, ethyl ester, propyl ester, n-butyl ester, isobutyl ester of acrylic acid, methacrylic acid, fumaric acid, maleic acid or the like. In particular, acrylic acid ester and methacrylic acid ester are preferable. Among these, the ionomer resin preferably includes the metal ion-neutralized product of the binary copolymer composed of ethylene-(meth)acrylic acid and/or the metal ion-neutralized product of the ternary copolymer composed of ethylene, (meth)acrylic acid, and (meth)acrylic acid ester.

Specific examples of the ionomer resins include trade name “Himilan (registered trademark) (e.g. the binary copolymerized ionomer such as Himilan 1555 (Na), Himilan 1557 (Zn), Himilan 1605 (Na), Himilan 1706 (Zn), Himilan 1707 (Na), Himilan AM3711 (Mg); and the ternary copolymerized ionomer such as Himilan 1856 (Na), Himilan 1855 (Zn))” commercially available from Du Pont-Mitsui Polychemicals Co., Ltd.

Further, examples include “Surlyn (registered trademark) (e.g. the binary copolymerized ionomer such as Surlyn 8945 (Na), Surlyn 9945 (Zn), Surlyn 8140 (Na), Surlyn 8150 (Na), Surlyn 9120 (Zn), Surlyn 9150 (Zn), Surlyn 6910 (Mg), Surlyn 6120 (Mg), Surlyn 7930 (Li), Surlyn 7940 (Li), Surlyn AD8546 (Li); and the ternary copolymerized ionomer such as Surlyn 8120 (Na), Surlyn 8320 (Na), Surlyn 9320 (Zn), Surlyn 6320 (Mg), HPF 1000 (Mg), HPF 2000 (Mg))” commercially available from E.I. du Pont de Nemours and Company.

Further, examples include “lotek (registered trademark) (e.g. the binary copolymerized ionomer such as lotek 8000 (Na), lotek 8030 (Na), lotek 7010 (Zn), lotek 7030 (Zn); and the ternary copolymerized ionomer such as lotek 7510 (Zn), lotek 7520 (Zn))” commercially available from ExxonMobil Chemical Corporation.

It is noted that Na, Zn, Li, and Mg described in the parentheses after the trade names indicate metal types of neutralizing metal ions for the ionomer resins. The ionomer resins may be used solely or in combination of at least two of them.

The cover composition constituting the cover of the golf ball of the present invention preferably includes, as a resin component, a thermoplastic polyurethane elastomer or an ionomer rein. In case of using the ionomer rein, it is preferred to use a thermoplastic styrene elastomer together. The content of the polyurethane or ionomer resin in resin component of the cover composition is preferably 50 mass % or more, more preferably 60 mass % or more, and even more preferably 70 mass % or more.

In the present invention, the cover composition may further contain a pigment component such as a white pigment (for example, titanium oxide), a blue pigment, and a red pigment; a weight adjusting agent such as zinc oxide, calcium carbonate, and barium sulfate; a dispersant; an antioxidant; an ultraviolet absorber; a light stabilizer; a fluorescent material or a fluorescent brightener; and the like, as long as they do not impair the effect of the present invention.

The amount of the white pigment (for example, titanium oxide) is preferably 0.5 part or more, more preferably 1 part or more, and the content of the white pigment is preferably 10 parts or less, more preferably 8 parts or less, with respect to 100 parts of the resin component constituting the cover by mass. If the amount of the white pigment is 0.5 part by mass or more, it is possible to impart the opacity to the resultant cover. Further, if the amount of the white pigment is more than 10 parts by mass, the durability of the resultant cover may deteriorate.

The slab hardness of the cover composition is preferably set in accordance with the desired performance of the golf balls. For example, in case of a so-called distance golf ball which focuses on a flight distance, the cover composition preferably has a slab hardness of 50 or more, more preferably 55 or more, and preferably has a slab hardness of 80 or less, more preferably 70 or less in shore D hardness. If the cover composition has a slab hardness of 50 or more, the obtained golf ball has a high launch angle and low spin rate on driver shots and iron shots, and thus the flight distance becomes large. If the cover composition has a slab hardness of 80 or less, the golf ball excellent in durability is obtained.

Further, in case of a so-called spin golf ball which focuses on controllability, the cover composition preferably has a slab hardness of less than 50, and preferably has a slab hardness of 20 or more, more preferably 25 or more in shore D hardness. If the cover composition has a slab hardness of less than 50, the flight distance on driver shots can be improved by the core of the present invention, as well as the obtained golf ball readily stops on the green due to the high spin rate on approach shots. If the cover composition has a slab hardness of 20 or more, the abrasion resistance improves. In case of a plurality of cover layers, the slab hardness of the cover composition constituting each layer can be identical or different, as long as the slab hardness of each layer is within the above range.

An embodiment for molding a cover is not particularly limited, and includes an embodiment which comprises injection molding the cover composition directly onto the core, or an embodiment which comprises molding the cover composition into a hollow-shell, covering the core with a plurality of the hollow-shells and subjecting the core with a plurality of the hollow shells to the compression-molding (preferably an embodiment which comprises molding the cover composition into a half hollow-shell, covering the core with the two half hollow-shells, and subjecting the core with the two half hollow-shells to the compression-molding).

When molding the cover in a compression molding method, molding of the half shell can be performed by either compression molding method or injection molding method, and the compression molding method is preferred. The compression-molding of the cover composition into half shell can be carried out, for example, under a pressure of 1 MPa or more and 20 MPa or less at a temperature of −20° C. or more and 70° C. or less relative to the flow beginning temperature of the cover composition. By performing the molding under the above conditions, a half shell having a uniform thickness can be formed. Examples of a method for molding the cover using half shells include compression molding by covering the core with two half shells. The compression molding of half shells into the cover can be carried out, for example, under a pressure of 0.5 MPa or more and 25 MPa or less at a temperature of −20° C. or more and 70° C. or less relative to the flow beginning temperature of the cover composition. By performing the molding under the above conditions, a golf ball cover having a uniform thickness can be formed.

In the case of directly injection molding the cover composition, the cover composition extruded in the pellet form beforehand may be used for injection molding, or the cover materials such as the base resin components and the pigment may be dry blended, followed by directly injection molding the blended materials. It is preferred to use upper and lower molds having a spherical cavity and pimples for forming a cover, wherein a part of the pimples also serves as a retractable hold pin. When molding the cover by injection molding, the hold pin is protruded to hold the core, and the cover composition which has been heated and melted is charged and then cooled to obtain a cover. For example, it is preferred that the cover composition heated and melted at the temperature ranging from 200° C. to 250° C. is charged into a mold held under the pressure of 9 MPa to 15 MPa for 0.5 to 5 seconds, and after cooling for 10 to 60 seconds, the mold is opened and the golf ball with the cover molded is ejected from the mold.

The concave portions called “dimple” are usually formed on the surface of the cover. The total number of the dimples is preferably 200 or more and 500 or less. If the total number is less than 200, the dimple effect is hardly obtained. On the other hand, if the total number exceeds 500, the dimple effect is hardly obtained because the size of the respective dimples is small. The shape (shape in a plan view) of dimples includes, for example, without limitation, a circle, polygonal shapes such as roughly triangular shape, roughly quadrangular shape, roughly pentagonal shape, roughly hexagonal shape, and another irregular shape. The shape of the dimples is employed solely or at least two of them may be used in combination.

In the present invention, the thickness of the cover of the golf ball is preferably 4.0 mm or less, more preferably 3.0 mm or less, even more preferably 2.0 mm or less. If the thickness of the cover is 4.0 mm or less, the resilience and shot feeling of the obtained golf ball become better. The thickness of the cover is preferably 0.3 mm or more, more preferably 0.5 mm or more, and even more preferably 0.8 mm or more, and most preferably 1.0 mm or more. If the thickness of the cover is less than 0.3 mm, the durability and the wear resistance of the cover may deteriorate. If the cover has a plurality of layers, it is preferred that the total thickness of the cover layers falls within the above range.

After the cover is molded, the mold is opened and the golf ball body is ejected from the mold, and as necessary, the golf ball body is preferably subjected to surface treatments such as deburring, cleaning, and sandblast. If desired, a paint film or a mark may be formed. The paint film preferably has a thickness of, but not limited to, 5 μm or larger, and more preferably 7 μm or larger, and preferably has a thickness of 50 μm or smaller, and more preferably 40 μm or smaller, even more preferably 30 μm or smaller. If the thickness is smaller than 5 μm, the paint film is easy to wear off due to continued use of the golf ball, and if the thickness is larger than 50 μm, the effect of the dimples is reduced, resulting in lowering flying performance of the golf ball.

When the golf ball of the present invention has a diameter in a range from 40 mm to 45 mm, a compression deformation amount of the golf ball (shrinking amount of the golf ball in the compression direction thereof) when applying a load from an initial load of 98 N to a final load of 1275 N to the golf ball is preferably 2.0 mm or more, more preferably 2.4 mm or more, even more preferably 2.5 mm or more, most preferably 2.8 mm or more, and is preferably 5.0 mm or less, more preferably 4.5 mm or less. If the compression deformation amount is 2.0 mm or more, the golf ball does not become excessively hard, and thus exhibits the good shot feeling. On the other hand, if the compression deformation amount is 5.0 mm or less, the resilience is enhanced.

The golf ball construction is not limited, as long as the golf ball of the present invention comprises a spherical core and at least one cover layer covering the spherical core. FIG. 1 is a partially cutaway sectional view showing the golf ball 2 according to the preferable embodiment of the present invention. The golf ball 2 comprises a spherical core 4, and a cover 12 covering the spherical core 4. Plurality of dimples 14 are formed on a surface of the cover. Other portions than dimples 14 on the surface of the golf ball 2 are land 16. The golf ball 2 is provided with a paint layer and a mark layer outside the cover 12, but these layers are not depicted.

The spherical core preferably has a single layered structure. Unlike the multi-layered structure, the spherical core of the single layered structure does not have an energy loss at the interface of the multi-layered structure when hitting, and thus has an enhanced resilience. The cover has a structure of at least one layer, for example a single layered structure, or a multi-layered structure of at least two layers. The golf ball of the present invention includes, for example, a two-piece golf ball comprising a spherical core and a single layered cover disposed around the spherical core, a multi-piece golf ball comprising a spherical core, and at least two cover layers disposed around the spherical core (including the three-piece golf ball), and a wound golf ball comprising a spherical core, a rubber thread layer which is formed around the spherical core, and a cover disposed over the rubber thread layer. The present invention can be suitably applied to any one of the above golf balls.

EXAMPLES

Hereinafter, the present invention will be described in detail by way of example. The present invention is not limited to examples described below. Various changes and modifications can be made without departing from the spirit and scope of the present invention.

[Evaluation Methods] (1) Compression Deformation Amount (mm)

A compression deformation amount of the core or golf ball (a shrinking amount of the core or golf ball in the compression direction thereof), when applying a load from 98 N as an initial load to 1275 N as a final load to the core or golf ball, was measured.

(2) Slab Hardness (Shore D Hardness)

Sheets with a thickness of about 2 mm were produced by injection molding the cover composition, and stored at 23° C. for two weeks. Three or more of these sheets were stacked on one another so as not to be affected by the measuring substrate on which the sheets were placed, and the hardness of the stack was measured with a type P1 auto loading durometer manufactured by Kobunshi Keiki Co., Ltd., provided with a Shore D type spring hardness tester prescribed in ASTM-D2240.

(3) Hardness Distribution of Spherical Core (JIS-C Hardness)

A type P1 auto loading durometer manufactured by Kobunshi Keiki Co., Ltd., provided with a JIS-C type spring hardness tester was used to measure the hardness of the spherical core. The hardness measured at the surface of the spherical core was adopted as the surface hardness of the spherical core. The spherical core was cut into two hemispheres to obtain a cut plane, and the hardness were measured at the central point and at predetermined distances from the central point. The core hardness were measured at 4 points at predetermined distances from the central point of the cut plane of the core. The core hardness was calculated by averaging the hardness measured at 4 points.

(4) Flight Distance (m) and Spin Rate (Rpm) on a Driver Shot

A metal-headed W#1 driver (XXIO S, loft: 11°, manufactured by Dunlop Sports Limited) was installed on a swing robot M/C manufactured by Golf Laboratories, Inc. A golf ball was hit at a head speed of 40 m/sec, and the flight distance (the distance from the launch point to the stop point) and the spin rate right after hitting the golf ball were measured. This measurement was conducted twelve times for each golf ball, and the average value was adopted as the measurement value for the golf ball. A sequence of photographs of the hit golf ball were taken for measuring the spin rate (rpm) right after hitting the golf ball. In tables, the flight distance and spin rate on the driver shots of golf balls No. 1 to No. 8 are shown as the difference from those of the golf ball No. 3, those of golf balls No. 9 to No. 16 are shown as the difference from those of the golf ball No. 13, those of the golf ball No. 18 is shown as the difference from those of the golf ball No. 17, those of golf balls No. 20 to No. 22 are shown as the difference from those of the golf ball No. 19, those of the golf ball No. 24 is shown as the difference from those of the golf ball No. 23, those of the golf balls No. 26 to No. 28 are shown as the difference from those of the golf ball No. 25, those of the golf ball No. 30 is shown as the difference from those of the golf ball No. 29, those of golf balls No. 32 to No. 34 are shown as the difference from those of the golf ball No. 31, and those of the golf ball No. 36 is shown as the difference from those of the golf ball No. 35.

[Production of Golf Balls] (1) Production of Cores

The rubber compositions having formulations shown in Tables 1 to 8 were kneaded and heat-pressed in upper and lower molds, each having a hemispherical cavity, at 170° C. for 20 minutes to prepare spherical cores having a diameter of 39.8 mm.

TABLE 1 Golf ball No. 1 2 3 4 Rubber BR730 100 100 100 100 composition Sanceler SR 25 29 23 27 (parts by mass) 2-thionaphthol 0.2 0.2 — 0.1 Zinc octanoate 5 7.5 — — Zinc stearate — — — — Stearic acid — — — — Zinc oxide 5 5 5 5 Barium sulfate *1) *1) *1) *1) Perhexyne 25B 0.49 0.49 — — Dicumyl peroxide — — 0.8 0.8 Core Center hardness 48.7 45.1 56.8 56.2 hardness 12.5% point hardness 52.4 50.3 60.7 62.7 distribution 25% point hardness 56.6 54.3 64.5 67.1 (JIS-C) 37.5% point hardness 61.8 56.5 66.5 68.3 50% point hardness 67.9 62.0 67.2 68.5 62.5% point hardness 72.0 69.3 67.6 68.2 75% point hardness 73.6 70.6 71.3 71.6 87.5% point hardness 70.5 69.5 72.1 75.4 Surface hardness 70.9 70.0 80.6 83.9 Surface hardness - center hardness 22.2 24.9 23.8 27.7 Core compression deformation amount (mm) 4.24 4.20 4.09 4.06 Cover composition A A A A Cover hardness (Shore D) 65 65 65 65 Cover thickness (mm) 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) −90 −100 0 0 Driver flying distance (m) 3.0 2.8 0 0.7 Compression deformation amount (mm) 3.54 3.50 3.39 3.23

TABLE 2 Golf ball No. 5 6 7 8 Rubber BR730 100 100 100 100 composition Sanceler SR 23 23 23 27 (parts by mass) 2-thionaphthol 0.2 0.2 0.32 0.2 Zinc octanoate — — — — Zinc stearate 10 — — — Stearic acid — 10 10 — Zinc oxide 5 5 5 5 Barium sulfate *1) *1) *1) *1) Perhexyne 25B 0.49 0.49 — 0.49 Dicumyl peroxide — — 0.8 — Core Center hardness 52.8 56.1 57.0 54.5 hardness 12.5% point hardness 56.6 59.1 62.0 60.4 distribution 25% point hardness 60.3 62.1 66.0 63.3 (JIS-C) 37.5% point hardness 64.9 64.6 67.0 64.2 50% point hardness 69.2 67.3 68.0 65.6 62.5% point hardness 72.0 69.4 70.0 70.2 75% point hardness 73.2 71.0 75.0 73.8 87.5% point hardness 71.2 68.5 76.0 73.0 Surface hardness 70.7 70.0 81.0 73.5 Surface hardness - center hardness 17.9 13.9 24.0 19.0 Core compression deformation amount (mm) 4.10 4.15 4.14 4.17 Cover composition A A A A Cover hardness (Shore D) 65 65 65 65 Cover thickness (mm) 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) −60 −50 −20 0 Driver flying distance (m) 2.2 2.0 0.8 0 Compression deformation amount (mm) 3.40 3.45 3.44 3.47

TABLE 3 Golf ball No. 9 10 11 12 Rubber BR730 100 100 100 100 composition Sanceler SR 29 27 29 26 (parts by mass) 2-thionaphthol 0.2 0.1 0.2 0.2 Benzoic acid 5.0 20.0 — — Zinc dibenzoate — — — 6.5 4-dimethylaminobenzoic acid — — 5.0 — Zinc oxide 5 5 5 5 Barium sulfate *1) *1) *1) *1) Perhexyne 25B 0.49 0.49 0.49 0.49 Dicumyl peroxide — — — — Core Center hardness 46.2 49.5 47.1 48.4 hardness 12.5% point hardness 50.2 52.2 50.1 51.2 distribution 25% point hardness 54.0 55.0 53.1 55.1 (JIS-C) 37.5% point hardness 56.3 59.3 56.3 59.1 50% point hardness 61.3 64.1 62.2 66.6 62.5% point hardness 70.6 67.2 69.5 71.6 75% point hardness 77.7 71.0 74.1 73.4 87.5% point hardness 77.1 66.7 73.0 69.5 Surface hardness 75.7 69.6 73.3 72.7 Surface hardness − center hardness 29.5 20.1 26.2 24.4 Core compression deformation amount (mm) 4.04 3.99 4.15 4.16 Cover composition A A A A Cover hardness (Shore D) 65 65 65 65 Cover thickness (mm) 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) −120 −50 −100 −80 Driver flying distance (m) 3.5 1.5 3.0 3.0 Compression deformation amount (mm) 3.34 3.29 3.45 3.46

TABLE 4 Golf ball No. 13 14 15 16 Rubber BR730 100 100 100 100 composition Sanceler SR 23 27 28 27 (parts by mass) 2-thionaphthol — 0.1 0.1 0.2 Benzoic acid — — 20.0 — Zinc dibenzoate — — — — 4-dimethylaminobenzoic acid — — — — Zinc oxide 5 5 5 5 Barium sulfate *1) *1) *1) *1) Perhexyne 25B — — — 0.49 Dicumyl peroxide 0.8 0.8 0.8 — Core Center hardness 56.8 56.2 49.5 54.5 hardness 12.5% point hardness 60.7 62.7 55.0 60.4 distribution 25% point hardness 64.5 67.1 61.8 63.3 (JIS-C) 37.5% point hardness 66.5 68.3 64.5 64.2 50% point hardness 67.2 68.5 67.6 65.6 62.5% point hardness 67.6 68.2 72.4 70.2 75% point hardness 71.3 71.6 75.3 73.8 87.5% point hardness 72.1 75.4 71.9 73.0 Surface hardness 80.6 83.9 75.9 73.5 Surface hardness − center hardness 23.8 27.7 26.4 19.0 Core compression deformation amount (mm) 4.09 4.06 3.99 4.17 Cover composition A A A A Cover hardness (Shore D) 65 65 65 65 Cover thickness (mm) 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) 0 0 −20 0 Driver flying distance (m) 0 0.7 0.5 0 Compression deformation amount (mm) 3.39 3.23 3.29 3.47

TABLE 5 Golf ball No. 17 18 19 20 21 22 Rubber BR730 100 100 100 100 100 100 composition Sanceler SR 29 25 25 25 25 25 (parts by Zinc oxide 5 5 5 5 5 5 mass) Barium sulfate *1) *1) *1) *1) *1) *1) 2-thionaphthol 0.2 — 0.2 — — — PBDS — — — 0.63 — — 2,6-DCP — 0.22 — — 0.22 — PCTP — — — — — 0.35 Zinc octanoate 5 5 5 5 5 5 Perhexyne 25B — 0.49 0.49 0.49 0.49 0.49 Dicumyl peroxide 0.8 — — — — — Core Center hardness 46.4 44.4 48.7 44.6 44.4 44.6 hardness 12.5% point hardness 53.8 48.7 52.4 49.4 48.7 47.7 distribution 25% point hardness 59.2 53.6 56.6 53.6 53.6 52.9 (JIS-C) 37.5% point hardness 62.2 57.2 61.8 56.3 57.2 56.9 50% point hardness 63.5 63.7 67.9 62.1 63.7 64.0 62.5% point hardness 68.3 71.6 72.0 71.1 71.6 71.8 75% point hardness 74.0 74.1 73.6 73.7 74.1 74.4 87.5% point hardness 74.6 72.4 70.5 70.9 72.4 72.7 Surface hardness 79.3 74.0 70.9 74.2 74.0 74.6 Surface hardness − center hardness 32.9 29.6 22.2 29.6 29.6 30.0 Core coefficient of restitution 0.000 0.002 0.000 0.005 0.007 0.002 Core compression deformation amount (mm) 4.33 4.39 4.24 4.36 4.39 4.23 Cover composition A A A A A A Cover hardness (Shore D) 65 65 65 65 65 65 Cover thickness (mm) 1.5 1.5 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) 0 −100 0 0 0 0 Driver flying distance (m) 0.0 3.5 0.0 1.0 1.0 0.5 Coefficient of restitution 0.000 0.002 0.000 0.005 0.007 0.002 Compression deformation amount (mm) 3.63 3.69 3.54 3.66 3.69 3.53

TABLE 6 Golf ball No. 23 24 25 26 27 28 Rubber BR730 100 100 100 100 100 100 composition Sanceler SR 27 24 23 23 24 23 (parts by Zinc oxide 5 5 5 5 5 5 mass) Barium sulfate *1) *1) *1) *1) *1) *1) 2-thionaphthol 0.2 — 0.2 — — — PBDS — — — 0.63 — — 2,6-DCP — 0.22 — — 0.22 — PCTP — — — — — 0.35 Zinc stearate 10 10 10 10 10 10 Perhexyne 25B — 0.49 0.49 0.49 0.49 0.49 Dicumyl peroxide 0.8 — — — — — Core Center hardness 48.8 51.5 52.8 48.1 51.5 48.8 hardness 12.5% point hardness 54.1 51.3 56.6 51.6 54.3 52.3 distribution 25% point hardness 59.5 51.9 60.3 57.2 57.9 57.0 (JIS-C) 37.5% point hardness 63.1 61.3 64.9 62.2 62.3 61.5 50% point hardness 65.0 67.3 69.2 66.5 67.3 66.2 62.5% point hardness 70.3 70.6 72.0 69.7 70.6 69.6 75% point hardness 75.6 72.2 73.2 71.0 72.2 71.0 87.5% point hardness 76.9 70.3 71.2 67.7 70.3 69.3 Surface hardness 81.0 71.8 70.7 71.3 71.8 71.2 Surface hardness − center hardness 32.2 20.3 17.9 23.2 20.3 22.4 Core coefficient of restitution 0.000 0.003 0.000 0.002 0.008 0.005 Core compression deformation amount (mm) 4.05 4.30 4.10 4.39 4.30 4.36 Cover composition A A A A A A Cover hardness (Shore D) 65 65 65 65 65 65 Cover thickness (mm) 1.5 1.5 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) 0 −100 0 0 0 0 Driver flying distance (m) 0.0 3.0 0.0 0.5 1.0 0.5 Coefficient of restitution 0.000 0.003 0.000 0.002 0.008 0.005 Compression deformation amount (mm) 3.35 3.60 3.40 3.69 3.60 3.66

TABLE 7 Golf ball No. 29 30 31 32 Rubber BR730 100 100 100 100 composition Sanceler SR 35 30 31 34 (parts by mass) Zinc oxide 5 5 5 5 Barium sulfate *1) *1) *1) *1) 2-thionaphthol 0.2 — 0.2 — PBDS — 0.63 — — 2,6-DCP — — — — PCTP — — — 0.35 Benzoic acid 5 5 5.2 5.2 Zinc dibenzoate — — — — Perhexyne 25B — 0.49 0.49 0.49 Dicumyl peroxide 0.8 — — — Core Center hardness 43.6 44.2 44.1 47.3 hardness 12.5% point hardness 48.7 47.6 48.2 50.8 distribution 25% point hardness 55.0 51.1 52.2 55.0 (JIS-C) 37.5% point hardness 58.8 53.4 54.1 55.9 50% point hardness 61.3 56.3 56.2 57.6 62.5% point hardness 61.4 65.2 64.0 61.2 75% point hardness 72.6 77.8 75.5 76.7 87.5% point hardness 80.2 79.5 77.9 82.2 Surface hardness 87.1 79.1 79.0 82.1 Surface hardness − center hardness 43.5 34.9 34.9 34.8 Core coefficient of restitution 0.000 0.002 0.000 0.006 Core compression deformation amount (mm) 4.30 4.30 4.00 3.90 Cover composition A A A A Cover hardness (Shore D) 65 65 65 65 Cover thickness (mm) 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) 0 −100 0 0 Driver flying distance (m) 0.0 4.0 0.0 1.0 Coefficient of restitution 0.000 0.002 0.000 0.006 Compression deformation amount (mm) 3.60 3.60 3.30 3.20

TABLE 8 Golf ball No. 33 34 35 36 Rubber BR730 100 100 100 100 composition Sanceler SR 37 34 25 29 (parts by mass) Zinc oxide 5 5 5 5 Barium sulfate *1) *1) *1) *1) 2-thionaphthol — — 0.2 — PBDS 0.63 — — 0.63 2,6-DCP — 0.22 — — PCTP — — — — Benzoic acid 5.2 5.2 — — Zinc dibenzoate — — 6.5 6.5 Perhexyne 25B 0.49 0.49 0.49 0.49 Dicumyl peroxide — — — — Core Center hardness 45.6 45.2 44.4 42.4 hardness 12.5% point hardness 48.7 49.2 48.8 47.3 distribution 25% point hardness 53.1 53.7 53.7 51.7 (JIS-C) 37.5% point hardness 55.3 55.7 56.5 54.5 50% point hardness 55.7 56.4 57.6 55.6 62.5% point hardness 58.5 61.4 67.8 65.8 75% point hardness 71.0 74.3 79.2 77.7 87.5% point hardness 80.4 81.5 79.2 77.6 Surface hardness 82.5 83.4 80.3 78.6 Surface hardness − center hardness 36.9 38.2 35.9 36.2 Core coefficient of restitution 0.008 0.010 0.000 0.002 Core compression deformation amount (mm) 3.90 4.00 4.10 4.20 Cover composition A A A A Cover hardness (Shore D) 65 65 65 65 Cover thickness (mm) 1.5 1.5 1.5 1.5 Ball Driver spin rate (rpm) 0 0 0 0 Driver flying distance (m) 1.0 0.0 0.0 0.4 Coefficient of restitution 0.008 0.010 0.000 0.002 Compression deformation amount (mm) 3.20 3.30 3.40 3.50 *1) In tables No. 1 to 8, as to an amount of barium sulfate, adjustment was made such that the golf ball had a mass of 45.4 g. BR730: a high-cis polybutadiene (cis-1,4 bond content = 96 mass %, 1,2-vinyl bond content = 1.3 mass %, Moony viscosity (ML₁₊₄ (100° C.) = 55, molecular weight distribution (Mw/Mn) = 3) available from JSR Corporation Sanceler SR: zinc acrylate (product of 10 mass % stearic acid coating) available from Sanshin Chemical Industry Co., Ltd. 2-thionaphthol: available from Tokyo Chemical Industry Co., Ltd. PBDS: bis(pentabromophenyl)disulfide 2,6-DCP: 2,6-dichlorothiophenol PCTP: pentachlorothiophenol Zinc octanoate: available from Mitsuwa Chemicals Co., Ltd. (purity of 99% or higher) Zinc stearate: available from Wako Pure Chemical Industries, Ltd. (purity of 99% or higher) Stearic acid: available from Tokyo Chemical Industry Co., Ltd. (purity of 98% or higher) Benzoic acid: available from Sigma-Aldrich (purity of 99.5% or higher) Zinc dibenzoate: available from Wako Pure Chemical Industries, Ltd. (purity of 95% or higher) 4-dimethylamino benzoic acid: available from Tokyo Chemical Industry Co., Ltd. (purity of 98% or higher) Zinc oxide: “Ginrei R” manufactured by Toho Zinc Co., Ltd. Barium sulfate: “Barium sulfate BD” manufactured by Sakai Chemical Industry Co., Ltd., adjustment was made such that the finally obtained golf ball had a mass of 45.4 g. Perhexyne 25B: 2,5-dimethyl-2,5-bis(t-butylperoxy)hexyne-3 Dicumyl peroxide: “PERCUMYL ® D” available from NOF Corporation.

(2) Production of Cover

Cover materials shown in Table 9 were mixed with a twin-screw kneading extruder to prepare the cover compositions in the pellet form. The extruding conditions of the cover composition were a screw diameter of 45 mm, a screw rotational speed of 200 rpm, and screw L/D=35, and the mixtures were heated to 150 to 230° C. at the die position of the extruder. The cover compositions obtained above were injection-molded onto the spherical cores to produce the golf balls having the spherical core and the cover covering the spherical core.

TABLE 9 Cover composition No. A Himilan 1605 50 Himilan 1706 50 Titanium oxide 4 Slab hardness (Shore D) 65 Formulation: parts by mass Himilan 1605: Sodium ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd Himilan 1706: Zinc ion neutralized ethylene-methacrylic acid copolymer ionomer resin available from Du Pont-Mitsui Polychemicals Co., Ltd

As apparent from the results of tables No. 1 to No. 8, the golf balls No. 1, 2, 5, 6, 9 to 12, 18 to 22, 24 to 28, 30 to 36 comprising a spherical core and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition containing (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) the specific organic peroxide, and (d) a carboxylic acid and/or a salt thereof, wherein the spherical core has a hardness difference between a surface hardness thereof and a center hardness thereof of 12 or more, have a low spin rate on driver shots, and thus travel a great flight distance, respectively.

The golf ball No. 7 is the case where the rubber composition contains (d) the carboxylic acid and/or the salt thereof and dicumyl peroxide as an organic sulfur peroxide. The golf ball No. 7 achieved the lower spin rate and greater flight distance compared with the golf ball No. 3, but the effect is smaller than that of Golf ball No. 6. The golf ball No. 8 is the case that the rubber composition does not contain (d) the carboxylic acid and/or the salt thereof, the effect of the lower spin rate and greater flight distance is not obtained.

The golf ball No. 15 is the case that dicumyl peroxide is used as an organic peroxide. The golf ball No. 15 achieved the lower spin rate and greater flight distance compared with the golf ball No. 13, but the effect is smaller than that of Golf balls No. 9 to No. 12. The golf ball No. 16 is the case where the rubber composition does not contain (d) the carboxylic acid and/or the salt thereof, the effect of the lower sin rate and greater flight distance is not obtained, compared with the golf ball No. 13.

Tables No. 5 to No. 8 indicates that golf balls having a spherical core and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition containing (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) the specific organic peroxide, (d) a carboxylic acid and/or a salt thereof and (f) the specific organic peroxide have a lower spin rate on driver shots and a high resilience, and thus travel a great distance.

The golf ball of the present invention has a low spin rate, and travels a great flight distance on driver shots. This application is based on Japanese Patent applications Nos. 2013-072719, 2013-072720, 2013-072721 filed on Mar. 29, 2013, the contents of which are hereby incorporated by reference. 

1. A golf ball having a spherical core and at least one cover layer covering the spherical core, wherein the spherical core is formed from a rubber composition containing (a) a base rubber, (b) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or a metal salt thereof as a co-crosslinking agent, (c) an organic peroxide represented by a following formula (1), and (d) a carboxylic acid and/or a salt thereof, provided that the rubber composition further contains (e) a metal compound in the case of containing only (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms as the co-crosslinking agent.

[In the formula (1), R¹ to R¹⁰ each represent a hydrogen atom, alkyl group, aryl group, aralkyl group, or alkylaryl group independently, wherein the alkyl group, aryl group, aralkyl group, or alkylaryl group may be bonded via an ester group.]
 2. The golf ball according to claim 1, wherein the rubber composition contains (d) the carboxylic acid and/or the salt thereof in a content ranging from 0.5 part to 40 parts by mass with respect to 100 parts by mass of (a) the base rubber.
 3. The golf ball according to claim 1, wherein (d) the carboxylic acid and/or the salt thereof includes an aliphatic carboxylic acid and/or a salt thereof.
 4. The golf ball according to claim 3, wherein the aliphatic carboxylic acid and/or the salt thereof has 1 to 30 carbon atoms.
 5. The golf ball according to claim 1, wherein (d) the carboxylic acid and/or the salt thereof includes an aromatic carboxylic acid and/or a salt thereof.
 6. The golf ball according to claim 5, wherein the aromatic carboxylic acid and/or the salt thereof includes a carboxylic acid having a benzene ring and/or a salt thereof.
 7. The golf ball according to claim 1, wherein R¹, R⁵, R⁶ and R¹⁹ each represent an alkyl group having 1 to 11 carbon atoms independently, and R² to R⁴ and R⁷ to R⁹ each represent an aryl group or an alkyl group having 1 to 11 carbon atoms independently.
 8. The golf ball according to claim 1, wherein (c) the organic peroxide includes 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3.
 9. The golf ball according to claim 1, wherein the rubber composition further comprises (f) an organic sulfur compound.
 10. The golf ball according to claim 9, wherein (f) the organic sulfur compound is at least one selected from the group consisting of thiophenols and/or metal salts thereof, thionaphthols and/or metal salts thereof, diphenylpolysulfides, and thiuram disulfides.
 11. The golf ball according to claim 9, wherein (f) the organic sulfur compound is at least one selected from the group consisting of thiophenols substituted with halogen, metal salts of the thiophenols substituted with halogen, and diphenyldisulfides substituted with halogen.
 12. The golf ball according to claim 9, wherein (f) the organic sulfur compound is at least one selected from the group consisting of 2,6-dichlorothiophenol, a metal salt of 2,6-dichlorothiophenol, pentachlorothiophenol, a metal salt of pentachlorothiophenol, pentabromothiophenol, a metal salt of pentabromothiophenol, bis(pentabromophenyl)disulfide, bis(2,6-dichlorophenyl)disulfide, bis(pentachlorophenyl)disulfide, 2-thionaphthol and a metal salt of 2-thionaphthol.
 13. The golf ball according to claim 9, wherein the rubber composition contains (f) the organic sulfur compound in a content ranging from 0.05 part to 5 parts by mass with respect to 100 parts by mass of (a) the base rubber.
 14. The golf ball according to claim 1, wherein the rubber composition contains (b) the metal salt of the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.
 15. The golf ball according to claim 1, wherein the rubber composition contains (b) the α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms and/or the metal salt thereof in a content ranging from 15 parts to 50 parts by mass with respect to 100 parts by mass of (a) the base rubber.
 16. The golf ball according to claim 1, wherein the spherical core has a hardness difference (Hs-Ho) between a surface hardness (Hs) thereof and a center hardness (Ho) thereof of 12 or more in JIS-C hardness.
 17. The golf ball according to claim 1, wherein the spherical core has a center hardness (Ho) in a range from 30 to 70 in JIS-C hardness.
 18. The golf ball according to claim 1, wherein the spherical core has a surface hardness (Hs) in a range from 65 to 100 in JIS-C hardness.
 19. The golf ball according to claim 1, wherein the spherical core has a compression deformation amount ranging from 2.0 mm to 6.0 mm, when applying a load from an initial load of 98N to a final load of 1275N to the spherical core having a diameter in a range from 34.8 mm to 42.2 mm.
 20. The golf ball according to claim 1, wherein (a) the base rubber includes a polybutadiene having a molecular weight distribution MW/MN (MW: weight-average molecular weight, MN: number-average molecular weight) in a range from 2.0 to 6.0. 